I have a Peavey ValveKing 20MH amp which has a single output jack for connecting it to a cabinet. I'm exploring my options for cabinets and am trying to understand if I can consider running cabinets in parallel or if I'm limited to running them in series.

Can I split a single output to run two cabinets in parallel, or will I need to run them in series?

It should be noted that two speakers in series will actually be quieter than each of them alone. Also, whereas increasing the speaker impedance is always safe for solid-state amps, I don't think this is true for tube power stages; at any rate these are reported to suffer damage when operated without load; too high impedance, even if it doesn't cause harm, can quite possibly mess up transmission properties of the power stage. So... it's rather not a good idea to put two or even more speakers in series, if you don't know the specs. Bottom line: you should always know the specs...
– leftaroundaboutMay 6 '16 at 18:07

Absolutely. I should have made it clearer that you should only operate a valve amplifier into an impedance corresponding to the selected output transformer tapping.
– user207421May 7 '16 at 0:37

A Peavey ValveKing 20MH amp has a speaker output impedance switch which allows selection of either 16Ω or 8Ω speaker/cabinet impedances with 8Ω max on the 20MH.

If you are considering wiring more than two speakers to the amplifier, you need to concern yourself with your amplifier’s ability to handle low impedance loads. If you ignore the way you wire them together you can damage the amplifier

Adding speakers in series increases the overall resistance of the circuit.

Zt =Za+Zb
8Ω speakers
Zt = 8Ω + 8Ω
Zt = 16Ω

Use 16Ω setting

Adding Speakers in parallel decreases the overall resistance of the circuit.

Zt =1(1/Za+1/Zb)
16Ω speakers
Zt =1(1/16Ω+1/16Ω)
= 1(1/8Ω)
Zt = 8Ω

Use 8Ω setting

As long as you're careful with output impedance you can run an 20 watt amp though any drivers greater than 20 watt. This will alows you to extend your clean headroom a bit, but any overdrive will be strictly from the natural tube amp breakup, your attack or your pedals.

Do not wire different speakers in series. The sound output of a speaker is specified with respect to its driving voltage, and speakers' impendance tends to vary wildly over its frequency range. The only thing you know is that it will not undercut its nominal impedance by more than 20%.

The amplifier is called "Valve King". Why? Valve power amplifiers don't actually have zero output impedance (which is the equivalent circuitry for a solid state amp: this does not mean that solid states can drive even 2Ohm speakers reliably but merely that as long as they do not break down, their output voltage will be unimpressed by the load).

Now I am totally guessing here, but the name suggests that the amplifier is from a time where tubes were special. That could mean that it only uses a preamp tube but has a solid state power amp. In that case, you would connect cabinets in parallel, making sure that the combined impedance is not lower than what the amplifier can work with. Putting 3 cabinets with 8 Ohm in parallel leaves you with 8/3 Ohm impedance, so it's 2.7Ohm. You need a 2 Ohm setting for accommodating that safely.

If your power amp stage is indeed tube, many bets are off. You then really want to connect a cabinet intended to match that particular amplifier.

You don't have to guess. ValveKings are still made by Peavey and they are 100% tube, so they do not have solid state power amps, they have tube power amps. Also, tubes are still special, which is why 100% tube designs are still be developed and manufactured by most amp manufacturers.
– Todd WilcoxMay 6 '16 at 21:06

The sound output of a speaker is specified with respect to its driving power. Never seen one that wasn't, in 50 years. You don't know any such thing as the 20% margin you mention. Some speakers dip far more than that.
– user207421May 7 '16 at 0:40